Magnetoresistive sensor having a bias field applied at approximately 56°

ABSTRACT

The present invention relates to a magnetic signal detection apparatus utilizing a ferromagnetic thin film magneto-resistive element which produces a maximum output of high linearity with an optimum direction of magnetization of the thin film magneto-resistive element by a small biasing magnetic field. A biasing magnetic field Hb is applied at an angle of approximately 56° to a direction of a current I along a longitudinal direction of a thin film magneto-resistive element (2) on a substrate (1) to make a direction of magnetization M to approximately 45° to attain a highly linear output between power supply terminals (4, 5).

TECHNICAL FIELD

The present invention relates to a magnetic signal detection apparatusutilizing a ferromagnetic thin film magneto-resistive element.

BACKGROUND ART

It has been known that a reproduced output and a reproductionsensitivity in converting a small signal magnetic field Hs to aresistance change in a resistive element are enhanced by using aferromagnetic thin film magneto-resistive element (MR element) as areproducing head and setting an angle θ to approximately 45°, where θ isan angle made between a direction of magnetization M of theferromagnetic thin film magneto-resistive element and a current Iflowing through the resistive element. An example of prior art in whicha biasing magnetic field Hb is applied in a direction of 45° to thedirection of the current I is shown in FIG. 8. In FIG. 8, by applyingthe biasing magnetic field Hb by a biasing magnet 3 in the direction ofapproximately 45° to the direction of the current I flowing through thethin film magneto-resistive element 2 as shown in a magneto-resistancecharacteristic of FIG. 9, an operating point at which high linearity andhigh reproduced output are attained is selected. In FIG. 8, numerals 4and 5 denote power terminals for supplying the current to the thin filmmagneto-resistive element 2.

It has also been known to arrange ferro-magnetic thin filmmagneto-resistive elements at an interval of one half of a magneticlattice pitch λ and apply a biasing magnetic field. In addition, amethod for applying the biasing magnetic field in the direction of 45°has been known as shown in FIG. 10 (JP-B-1-45008). In FIG. 10, numerals6 and 8 denote power terminals, numeral 7 denotes an output terminal,numeral 9 denotes a biasing magnetic field and numeral 10 denotes asignal magnetic field recording medium.

In any one of the above methods, the biasing magnetic field is appliedin the direction of 45° to a longitudinal direction (current direction)of the resistive element. When an MR film formed on a substrate (made ofceramic or glass material) is patterned to form a resistive element(having a film width of several μm.sub.˜ several tens μm)., thedirection of magnetization M of the resistive element is oriented in thelongitudinal direction of the resistive element by a shape effect. Inorder to orient the magnetization in the direction of 45° to thelongitudinal direction by the biasing magnetic field applied in thedirection of 45° to the longitudinal direction of the resistive element,it is necessary to apply a large biasing magnetic field in the order ofseveral thousands .sub.˜ ten thousands Gausses because a shape energypossessed by the resistive element is large. When a relatively smallbiasing magnetic field (several hundreds Gausses) is applied, themagnetization of the resistive element is oriented to a directionsmaller than 45°. Because of those phenomena, the following problemsarise.

(1) Because the magnetization of the resistive element is bound by thelarge biasing magnetic field or because the direction of magnetizationis smaller than 45°, a detection output of a small leakage magneticfield of a magnetic material bearing a magnetic signal to be detected issmall, a signal to noise ratio is small, a distortion increases and theprocessing by a signal processing circuit is difficult to attain.Recently, the compactness of the apparatus, the enhancement of thecontrol accuracy and the fine magnetic signal for high fidelityreproduction are required and hence the leakage magnetic field to bedetected is becoming more and more weak.

(2) It is necessary to bring the detection element as closely to themagnetic material bearing the magnetic signal as possible to detect in alarge area of the signal magnetic field to be detected in order toattain a large output. In order to detect a relative linear displacementor rotational displacement between the magnetic material to be detectedand the detection element by a uniform fine gap, it is necessary toprecisely machine a surface of the magnetic material and adjust the gapby using a gap sheet. This requires a time consuming manufacturingprocess.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a magnetic signaldetection apparatus which solves the above problems encountered in theprior art and allows the orientation of the magnetization M to ±45° or±135° relative to the current with a minimum biasing magnetic field.

In order to achieve the above object, the magnetic signal detectionapparatus of the present invention comprises an anisotropic resistiveelement arranged to face in a face-to-face fashion or perpendicularly toa magnetic signal recording surface on a substrate and made of aferromagnetic metal thin film, and a magnetic filed applicationapparatus for applying a biasing magnetic field of a predetermined angleto said resistive element. The direction of application of the biasingmagnetic field by said magnetic field application device is set to meetthe following formula to the direction of current flowing through saidresistive element:

    α=±(sin.sup.-1 ((Ku+Ks)/H×M)+φ)

where

φ: 45° or 135°

α: angle between current and biasing magnetic field

Ku: anisotropy energy (J/m³)

Ks: contour anisotropy energy (J/m³)

H: biasing magnetic field (A/m)

M: saturation magnetization (T)

More specifically, the biasing magnetic field to the resistive elementis no smaller than approximately ±47° and no larger than approximately±75° to the direction of the current or the biasing magnetic field tothe resistive element is no smaller than approximately ±105° and nolarger than approximately ±133° to the direction of the current.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a construction of a thin film magneto-resistive element inone embodiment of the present invention;

FIG. 2 shows an arrangement of a detection element to a magnetic signalrecording medium in other embodiment;

FIG. 3 shows an arrangement of the detection element to the magneticsignal recording medium in a further embodiment;

FIG. 4 shows a relation between a direction of a biasing magnetic fieldHb and a direction of magnetization M when the biasing magnetic field isapplied;

FIG. 5 illustrates a mid-point output voltage when the biasing magneticfield is applied in a direction of 56° to a direction of current;

FIG. 6 shows an angle of the biasing magnetic field and an angle of themagnetization by using a biasing magnetic field strength as a parameter;

FIG. 7 shows an arrangement illustrating a relation among a biasingmagnet, a substrate and an MR element when the biasing magnetic field isapplied by the biasing magnet;

FIG. 8 shows a construction of a prior art thin film magneto-resistiveelement;

FIG. 9 shows a characteristic chart of magneto-resistance when themagnetization M is oriented substantially 45°; and

FIG. 10 shows an arrangement of a prior art detection element to amagnetic signal recording medium.

BEST MODES OF CARRYING OUT THE INVENTION

The preferred embodiments of the present invention are now explainedwith reference to the drawings.

When a biasing magnetic field Hb is applied to a film having an axis ofeasy magnetization along a longitudinal direction of a ferromagneticthin film resistive element (direction of a current I) as shown in FIG.4, the direction of magnetization M is stabilized at a state in which atotal energy Et is minimum and an angle between the magnetization M andthe current I is determined. The total energy is a sum of an anisotropyenergy Ku×sin² θ, a magneto-static energy Ks×sin² θ owing to the contoureffect and a Zehmann energy -H×M×cos(θ-α), that is;

    Et=Ku×sin.sup.2 θ+Ks×sin.sup.2 θ-H×M×cos(α-θ)              (1)

where

Ku: anisotropy energy (J/m³)

Ks: contour anisotropy energy (J/m³)

M: saturation magnetization (T)

H: biasing magnetic field (A/m)

θ: angle between current and magnetization

α: angle between current and biasing magnetic field

A general relation between α and H when θ is ±45° or ±135° is given by:

    α=±(sin.sup.-1 ((Ku+Ks)/H×M)+φ)         (2)

where

φ: 45° or 135°

film material: NiFe (permaloy)

film thickness: 0.1 μm

film width: 20 μm

Placing Ku=200, Ks=2000 and M=1 in the formula (2), a relation between αand H is given by:

    α=±(sin.sup.-1 (2200/H)+45°)               (3)

The formula (3) indicating the relation of α and H is shown in FIG. 6,where H=Hb×80.

For example, assuming that the film thickness =0.1 μm, the film width=20 μm, the biasing magnetic field strength Hb=140 Oersteds (Oe) and thepermaloy film is used, the direction of application of Hb required toorient the magnetization M to a direction of substantially 45° to thelongitudinal direction (the direction of current) of the resistiveelement is approximately 56° to the longitudinal direction of theresistive element.

Assuming that a biasing magnetic field is applied in the direction ofsubstantially 45° in accordance with the prior art, the biasing magneticfield strength to orient the magnetization to substantially 45° in theferromagnetic thin film (MR film) is greater than approximately 10,000Oe. Similarly, when Hb is relatively weak, approximately 140 Oe and itis applied in the direction of 45° to the direction of current, themagnetization is oriented in the direction of approximately 33°. Anoutput when the magnetic signal is reproduced by the construction ofFIG. 2 by using the MR film having a film thickness of 0.1 μm and a filmwidth of 20 μm is calculated by using an angle between the magnetizationM and the current as a parameter. When the applied voltage Vcc=5.0 V,the magneto-resistance change rate Δρ/ρ=0.026 and the signal magneticfield Hs=10 Oe, the output voltage Vo=6.6 mV p-p with the biasingmagnetic field strength Hb=140 Oe and the biasing magnetic field angleα=56°. When the biasing magnetic field strength Hb=140 Oe and thebiasing magnetic field angle α=45°, Vo=6.2 mV p-p. It is understood thatthe former output is approximately 7% higher than the latter output. Inorder to orient the magnetization M to a direction of 45° when thebiasing magnetic field angle α=45°, it is necessary to apply the biasingmagnetic field having the strength Hb of 10,000 Oe or larger, and theoutput is approximately 0.1 mV p-p which exhibits substantial decrease.Accordingly, the reproduced output is larger when the Hb ofapproximately 140 Oe is applied in the direction of 56° so that themagnetization M is oriented in the direction of approximately ±45° (orapproximately ±135°). In the reproducing magnetic head with theconstruction shown in FIG. 1, the output is increased by applying thebiasing magnetic field having the strength Hb=140 Oe in the direction ofα=56°.

In this manner, by applying the weak magnetic field strength Hb in theangle closer to the right angle to the direction of current than to thetarget angle of magnetization M, the angle between the magnetization Mand the current is made to approximately ±45° (or approximately ±135°)so that the reproduction characteristic of the magnetic signal isimproved.

As shown in FIG. 5, when two resistive elements are connected in series,the output from a midpoint is represented by:

    K×Vcc×cos.sup.2 θ

where

K: proportional constant

Vcc: power supply voltage

θ: angle between magnetization M and current

and in principle, a temperature drift becomes zero when θ=45°. In thismanner, the reproduction characteristic of the magnetic signal isimproved by orienting the magnetization to a direction of ±45° or ±135°to the current direction.

As seen from FIG. 6, a relation between the biasing magnetic fieldstrength Hb and the biasing angle θ is such that when Hb is increased, adifference (θ-α) between the angle α of the biasing magnetic fieldstrength Hb and the angle θ of the magnetization M decreases.

As a currently available magnet material for the biasing magnet, aneodium group having a large remanent magnetic flux density Br and amaximum magnetic flux density Brmax of approximately 13 KG is known.When it is used in a manner shown in FIG. 7, an applied magnetic fluxstrength on the resistive element is approximately 1400 Oe at maximumbecause the magnetic field is applied to the thin film resistive elementthrough the thickness of the substrate (approximately 0.7 mm). The angleof the biasing magnetic field which results in approximately ±45° (or±135°) between the magnetization M and the current is, from the formula(3), approximately ±47° (or ±133°). On the other hand, as the appliedmagnetic field is decreased, the angle θ of the magnetization Msignificantly changes in the vicinity of 140 Oe. By taking thedemagnetization due to the temperature change into account, the appliedmagnetic field strength is preferably at least 140 Oe. When the appliedmagnetic field strength is 140 Oe, the angle of the biasing magneticfield is approximately ±56° (or approximately ±124°). Accordingly, whenthe neodium magnet is used and the MR film thickness is 0.05.sub.˜ 0.1μm and the film width is 8.sub.˜ 20 μm, the angle of the biasingmagnetic field is approximately ±47°.sub.˜ ±75° (or approximately±105°.sub.˜ ±133°).

Specifically, when NiFe is used as the MR film, the anisotropy energyKu=5×40, and when Hb of 140 Oe or 1400 Oe is applied to the MR filmhaving the film thickness of 0.05 μm.sub.˜ 0.1 μm and the film width of8 μm.sub.˜ 20 μm, the angle α for the maximum output is shown below.

    ______________________________________                                              Film                     α(°)                                                                    α(°)                             Thick-    Film           when   when                                    Ku    ness      Width   Ks     Hb = 140                                                                             Hb = 1400                               (J/m.sup.3)                                                                         (μm)   (μm) (J/m.sup.3)                                                                          Oe     Oe                                      ______________________________________                                        200   0.05      20      1000   51.15  45.61                                   200   0.1       20      2000   56.32  46.12                                   200   0.05       8      2500   58.94  46.38                                   200   0.1        8      5000   72.66  47.66                                   ______________________________________                                         Ks = film thickness/film width × 10.sup.4 × 40               

When NiCo is used as the MR film, the anisotropy energy Ku=15×40, andwhen Hb of 140 Oe or 1400 Oe is applied to the MR film having the filmthickness of 0.05 μm.sub.˜ 0.1 μm and the film width of 8 μm.sub.˜ 20μm, the angle α for the maximum output is shown below.

    ______________________________________                                              Film                     α(°)                                                                    α(°)                             Thick-    Film           when   when                                    Ku    ness      Width   Ks     Hb = 140                                                                             Hb = 1400                               (J/m.sup.3)                                                                         (μm)   (μm) (J/m.sup.3)                                                                          Oe     Oe                                      ______________________________________                                        600   0.05      20      1000   53.21  45.82                                   600   0.1       20      2000   58.42  46.33                                   600   0.05       8      2500   61.06  46.58                                   600   0.1        8      5000   75.0   47.86                                   ______________________________________                                         Ks = film thickness/film width × 10.sup.4 × 40               

Accordingly, when a currently available magnet is used and the MR filmhaving the film thickness of 0.05 μm.sub.˜ 0.1 μm and the film width of8 μm.sub.˜ 20 μm is used, the biassing angle of the biasing magnetapplied to the MR film is approximately ±47°.sub.˜ ±75°.

Specific embodiments of the present invention are now explained indetail with reference to the drawings.

FIG. 1 shows a construction of the thin film magneto-resistive elementin one embodiment in which a biasing magnetic field is applied in thedirection of 56° to the longitudinal direction (the direction of thecurrent I) to a detection element which faces a magnetic signalrecording medium. Unlike FIG. 8 which shows the prior art, the biasingmagnetic field Hb is applied by the biasing magnet 3 in the angle of 56°to the longitudinal direction (the direction of the current I) of thethin film magneto-resistive element 2 to orient the magnetization M tothe angle of 45° to the longitudinal direction.

FIG. 2 shows an arrangement of the detection element to the magneticsignal recording medium in other embodiment of the present invention.The thin film magneto-resistive elements are arranged at an interval ofλ/2 where λ is a magnetic lattice pitch and the biasing magnetic fieldHb is applied in the direction of approximately 56° so that thedirection of magnetization is approximately 45° to the longitudinaldirection of the resistive element.

FIG. 3 shows an arrangement of the detection elements to the magneticsignal recording medium in a further embodiment of the presentinvention. The resistive elements are arranged at an interval of λ andthey are divided into first group and second group with a separation of3λ/2 between the first resistive element group and the second resistiveelement group, and the biasing magnetic field Hb is applied in thedirection of approximately 56° so that the magnetization is oriented toapproximately 45° to the longitudinal direction of the resistiveelement.

The thin film which forms the resistive element is made of aferromagnetic material such as NiFe or NiCo having the film thickness of0.05 μm.sub.˜ 0.1 μm and the film width of 8 μm.sub.˜ 20 μm, and aplurality of pieces are arranged in a zigzag pattern. A biasing magneticfield of approximately 140 Oe is applied so that it is approximately 56°to the longitudinal direction (the direction of the current) of theresistive element. The magnet which serves as a magnetic fieldapplication device to apply the biasing magnetic field to the detectionelement is made of ferrite or rare earth group isotropic material and itis integrally mounted on a substrate of the detection element by resinmold or adhered by epoxy bond. Alternatively, for the biasing magnet, abiasing permanent magnetic film layer made of a high coercivity magneticfilm may be arranged in the vicinity of the detection element to applythe biasing magnetic field, or a current may be supplied to an inductorso that a magnetic field generated by the current is applied, or acurrent may be supplied to a conductor so that a magnetic fieldgenerated around the conductor is applied.

In the respective embodiments, the thin film magneto-resistive elementmay face perpendicularly to the magnetic signal recording medium insteadof face-to-face.

In accordance with the above embodiments, the biasing magnetic field maybe small and the magnet or coil for applying the biasing magnetic fieldmay be small and the apparatus may be small.

In accordance with the magnetic signal detection apparatus of thepresent invention, when the magnetic signal is to be detected from themagnetic signal recording medium by the magnetic detection element andthe magnetic field application device, the direction of the biasingmagnetic field is made closer to the right angle to the direction of thecurrent flowing through the resistive element so that the magnetizationM is oriented to ±45° or ±135° to the direction of the current and themaximum output with high linearity is attained with the small biasingmagnetic field, the magnetic signal reproduction characteristic isimproved, and the compactness of the entire apparatus is attained.

We claim:
 1. A magnetic signal detection apparatus comprising:ananisotropic resistive element arranged to face in a face-to-face fashionor perpendicularly to a magnetic signal recording surface on a substrateand made of a ferromagnetic metal thin film; and a magnetic fieldapplication apparatus for applying a biasing magnetic field of apredetermined angle to said resistive element; wherein the direction ofapplication of the biasing magnetic field by said magnetic fieldapplication device is set to meet the following formula to the directionof current flowing through said resistive element:

    α=±(sin.sup.-1 ((Ku+Ks)/H×M)+φ)

where φ is 45° or 135°, α is angle between current and biasing magneticfield, Ku is anisotropy energy, Ks is Contour anisotropy energy, H isbiasing magnetic field, and M is saturation magnetization.
 2. A magneticsignal detection apparatus according to claim 1 wherein the biasingmagnetic field to the resistive element is approximately ±47°.sub.˜approximately ±75° to the direction of the current.
 3. A magnetic signaldetection apparatus according to claim 1 wherein the biasing magneticfield to the resistive element is approximately ±105°.sub.˜approximately ±133° to the direction of the current.
 4. A magneticsignal detection apparatus according to claim 1 wherein a firstresistive element group having a plurality of resistive elementsconnected in series and a second resistive element group having aplurality of resistive elements connected in series are connected, anoutput terminal is provided at a junction of said resistive elementgroups, opposite ends to the junction are connected to power supplyterminals, the respective resistive elements are arranged face-to-faceto the magnetic signal recording surface with a magnetic lattice pitch λand the first resistive element group and the second resistive elementgroup are spaced by a distance equal to nλ/2 or (n/2+1/2)λ where n is aninteger.
 5. A magnetic signal detection apparatus comprising:ananisotropic resistive element arranged to face in a face-to-face fashionor perpendicularly to a magnetic signal recording surface on a substrateand made of a ferromagnetic metal thin film; and a magnetic filedapplication apparatus for applying a biasing magnetic field of apredetermined angle to said resistive element; wherein when a biasingmagnetic field strength by said magnetic field application device is 140to 1400 Oersteds, a film thickness of the thin film of said resistiveelement is 0.05.sub.˜ 0.1 μm and a film width is 8.sub.˜ 20 μm, thedirection of application of the biasing magnetic field by said magneticfield application device is approximately 47°.sub.˜ approximately 75° orapproximately 105°.sub.˜ approximately 133° to the direction of thecurrent flowing through said resistive element.
 6. A magnetic signaldetection apparatus according to claim 5 wherein said resistive elementis a nickel-cobalt alloy.
 7. A magnetic signal detection apparatusaccording to claim 5 wherein said resistive element is a nickel-ironalloy.
 8. A magnetic signal detection apparatus according to claim 5wherein said magnetic field application device is a ferrite magnet or arare earth group magnet.